Isovector radiative decays and second-class currents in mass 8 nuclei

The total isovector $M1\mathrm{}$ radiative width of the 16.6-16.9 MeV doublet in ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ to the broad first excited state has been measured via the ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha , \gamma )$ reaction to be 6.10±0.53 eV. Angular distributions at the 16.6 and 16.9 MeV resonances have been measured and determine the isovector $E2\mathrm{}$ to isovector $M1\mathrm{}$ mixing ratio to be $\delta \left[\right(\frac{E2, T=1)}{(M1, T=1)}]=0.19\mathrm{}\pm 0.03$. A comparison of these results with recent $\beta -\alpha$ angular correlation measurements of ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ is consistent with predictions of the conserved vector current theory and the absence of a second-class induced tensor form factor.